(define-record-type lit (fields datum))
(define-record-type varref (fields var))
(define-record-type function (fields formal body))
(define-record-type app (fields rator rand))

(define parse
  (lambda (datum)
    (cond
     ((number? datum) (make-lit datum))
     ((symbol? datum) (make-varref datum))
     ((pair? datum) (if (eq? (car datum) 'lambda)
                        (make-function (caadr datum) (parse (caddr datum)))
                        (make-app (parse (car datum)) (parse (cadr datum)))))
     (else (error 'parse "Invalid concrete syntax" datum)))))